The present invention relates to a device for converting infrared radiation into near infrared radiation or visible radiation, and more particularly, to a semiconductor device comprising multiple quantum wells in a forward-biased P-N structure, for directly converting incident infrared radiation in the range of approximately 4-15 .mu.m into near infrared radiation or visible radiation.
Infrared imaging is a topic of current interest due to potential military and commercial applications. Current infrared imaging technology consists of designing infrared photo-detector elements which are then set in an array to detect two dimensional images of infrared light. Electrical signals from each element of the array must be detected and processed before application to a separate display terminal where a visible representation of the infrared image is produced.
A typical infrared imaging converter is described by way of example, in U.S. Pat. No. 4,914,296 for "Infrared Converter" issued to Reinhold et al. as shown in FIG. 1, where infrared radiation IR emanating from a scene is converted by an infrared detector array 1 comprising a three-by-three array of sensitive elements into a beam of electrons, designated as "e", to be multiplied by an electron multiplier 3 via a first vacuum 2 and converted into visible radiation by a plurality of light-emitting-diodes LEDs 5 via a second vacuum 4. The process of electrically sampling each image element in a large array as disclosed by Reinhold et al. '296 is however, technically difficult and expensive.
Conventional infrared detectors such as Reinhold et al. '296 rely upon mercury cadmium telluride (HgCdTe) detectors and silicon semiconductor detectors for the detection of infrared radiation. A HgCdTe infrared detector is based on a P-I-N semiconductor and operates on the principle of detecting infrared photons by measuring electrons that are released from the valence band to the conduction band in the intrinsic layer from the absorption of the infrared photon energy.
Similarly, a silicon semiconductor detector also detects infrared radiation by the absorption of infrared photons that release the electrons into the conduction band to enable an electric field to direct the released electrons to specific terminal contacts where the released electrons can be measured. Both of these types of infrared detectors suffer from a number of drawbacks including the difficulties associated with current fabrication techniques, such as molecular beam epitaxy (MBE) and metal organochemical vapor deposition (MOCVD), and integration with other circuitry, such as read-out circuits in order to enable the detected photons to be imaged.
Current developments in the art of infrared detection utilize contemporary fabrication techniques such as MBE and MOCVD, and endeavor to provide devices such as quantum well infrared photodetectors (QWIP) that are adaptable to read-out circuitry for enabling an infrared image to be seen as a visible image.
A quantum well infrared photodetector is typically a device as shown in FIG. 2A that incorporates an array of aluminum gallium arsenide (AlGaAs) barrier layers 22 and appropriately doped gallium arsenide (GaAs) well layers 23, in an alternating pattern forming a multiple quantum well heterostructure between adjacent ohmic contact layers 21 and 25. Both the ohmic contact layers 21 and 25 as well as the alternating AlGaAs barrier layers 22 and GaAs well layers 23 are doped with n-type impurities. Blocking layer 24 may also be doped at an edge proximate to contact layer 25 in order to maximize the ohmic contact between contact layer 25 and the alternating AlGaAs barrier layers 22 and GaAs well layers 23. An energy diagram of a typical QWIP detector is illustrated in FIG. 2B, where dopant electrons "e" bound in the well structure formed by the conduction band CB of the GaAs well layers 23 acquire enough optical energy from absorbed incident infrared radiation to reach the conduction band CB of the AlGaAs barrier layers 22 in order to be free carriers measurable as induced photocurrent by infrared radiation. Dopant electrons can also be excited to give rise to free carriers by thermal energy with thermal activation energy which is approximately equal to the AlGaAs/GaAs conduction band offset.
In such QWIP detectors, dopant electrons "e-" are confined in the GaAs well layers 23 of a heterostructure quantum well between the two ohmic contact layers 21 and 25. Bias across the ohmic contacts 21 and 25 provides an electric field which, when the electrons "e-" are excited above the tops of the AlGaAs barriers 22, pushes the electrons toward the anode, giving rise to a photocurrent that is measured as an indication of the magnitude of the incident infrared radiation.
An example of a QWIP detector is U.S. Pat. No. 4,620,214 for "Multiple Quantum-Well Infrared Detector" issued to Margalit et al. which provides the multiple quantum-well structure with upper and lower ohmic contacts to bias the individual quantum-well layers in order to achieve photocurrent. Generally, the thickness of each quantum well layer of GaAs is usually small, generally about 20-60 Angstroms and most preferably about 40 Angstroms. The thickness of each barrier layer of AlGaAs is generally in the range of about 80-300 Angstroms. It is also preferred that each quantum well layer be heavily doped n-type with a donor impurities such as Ge, S, Si, Sn, Te or Se. Lattice match and thermal coefficient considerations, impurity concentrations and fabrication techniques for these QWIP detectors are well known in the art. The sensitivity of the QWIP detectors has also been improved by increasing the barrier layer of AlGaAs from about 300 Angstroms to about 500 Angstroms, while maintaining the thickness of the quantum well layer of GaAs of approximately 40 Angstroms as reported in Levine et al., "High Sensitivity Low Dark Current 10 .mu.m GaAs Quantum Well Infrared Photodetectors," Applied Physics Letters, Vol 56, No. 9, 26 Feb. 1990, pages 851-853. Such an increase in the barrier layer width of AlGaAs would reduce the tunneling dark current, which consists primarily of electrons tunneling through the barrier layer of AlGaAs between the ground states of adjacent quantum well layers of GaAs, thereby significantly increasing the level of sensitivity and quantum efficiency of the QWIP detectors.
Other techniques to improve the sensitivity of the QWIP detectors are disclosed, for example, in U.S. Pat. No. 5,077,593 for "Dark Current-Free Multiquantum Well Superlattice Infrared Detector," where Sato et al endeavors to keep the barrier layer of AlGaAs thin, but a thicker tunneling current blocking layer 24 at the end of the super-lattice in the path of the tunneling electrons as shown in FIG. 2A, in the range of approximately 800-3000 Angstroms in order to enhance the detector's quantum efficiency.
In typical QWIP detectors, the detection range of infrared radiation is usually in the range of between 4 to 15 .mu.m of wavelength, depending upon how the quantum well width and the barrier height of the QWIP detectors are constructed. One example of such construction is disclosed by U.S. Pat. No. 5,238,868 for "Bandgap Tuning Of Semiconductor Quantum Well Structures" issued to Elman et al. which contemplates an ion implantation technique for selectively tuning the bandgap of the quantum well layers to permit an accurate and precise modification of the geometrical shapes, width, barrier heights and composition of the quantum well in a spatially selective manner.
For the detection of long wavelength infrared radiation, designs such as those disclosed by Levine et al., "Bound-to-Extended State Absorption GaAs Superlattice Transport Infrared Detectors," J Applied Physics Letters, Vol 64, No. 3, 1 Aug. 1988, pages 1591-1593; "Broadband 8-12 .mu.m High-Sensitivity GaAs Quantum Well Infrared Photodetector," Applied Physics Letters, Vol. 54, No. 26, 26 Jun. 1989, pages 2704-2706; and "High-Detectivity D*=1.0.times.10.sup.10 cm/Hz/W GaAs/AlGaAs Multiquantum Well .lambda.=8.3 .mu.m Infrared Detector," Applied Physics Letters, Vol. 53, No. 4, 25 Jul. 1988, pages 296-298, can be employed.
For the detection of medium wavelength infrared radiation, U.S. Pat. No. 5,160,991 for "Electromagnetic Wave Detector And Image Analyzer Comprising A Detector Of This Type" issued to Delacourt et al. may be noted. Variations on the theme are disclosed by U.S Pat. No. 5,239,186 for "Composite Quantum Well Infrared Detector" issued to McIver et al. which envisions a multiple quantum well infrared detector having a wider infrared radiation bandwidth; and U.S. Pat. No. 5,036,371 for "Multiple Quantum Well Device" issued to Schwartz which contemplates one wide well of GaAs and a plurality of narrower wells spaced from each other and sandwiched between a plurality of barriers of AlGaAs in order to detect a desired spectral range.
Other improvements are disclosed, for example, in U.S. Pat. No. 5,198,682 for "Multiple Quantum Well Superlattice Infrared Detector With Graded Conductive Band" issued to Wu et al. which sought to establish the barrier energy level gradients of the quantum well barrier layers to produce an internal electric field within the AlAs/AlGaAs superlattice that facilitates the movement of photoexcited charge carriers through the superlattice, even in the absence of an external bias voltage from the upper and lower ohmic contacts.
In these structures, the infrared radiation absorption leads directly to a measured photocurrent; however none of the foregoing structures provides means for directly converting infrared radiation into visible radiation. U.S. Pat. No. 5,160,992 for "Device For The Conversion Of An Infrared Radiation Into Another Radiation Of Energy Greater Than That Of This Infrared Radiation" issued to Gerard et al. however, provides a heterostructure with a double quantum well employed as a P-N junction in order to absorb infrared radiation to emit visible radiation as a result of the recombination of excess electrons and holes created in the quantum well while the P-N junction is flat banded by an external optical source. A specific double quantum well structure is shown in FIG. 3A where two wells 32 and 34 of the same width and same depth are doped with impurity elements of N-type "donor" and P-type "acceptor," respectively, in order to create excess electrons and excess holes for the subsequent recombination process. Barrier layers of AlGaAs 31 and 35 are undoped and identical in character with each forming a potential barrier for the electrons and holes. Central AlGaAs barrier layer 33 is also undoped and forms an intermediate potential barrier of greater band-gap for containing the electrons and holes within the two GaAs wells 32 and 34 in a delocalized electronic state. FIG. 3B illustrates an energy diagram of the double quantum well structure as shown in FIG. 3A, where dopant electrons "e", as ionized donors, bound in the quantum well 32 are optically excited by the infrared radiation across the band-gap, from a state of the miniband e.sub.1, which state is principally localized within the quantum well 32, into a delocalized state of the miniband e.sub.3. These dopant electrons are then recombined with holes, or ionized acceptors (not shown), which are already present within the quantum well 34 to thereby generate visible radiation.
The efficiency of the absorption of the infrared radiation disclosed by Gerard et al. '992 is, however, inadequate. Although the absorption efficiency may be increased by increasing the number of pairs of quantum wells and forming a series of P-N structures in the same arrangement as was shown in FIG. 3A, the construction of such P-N structures comprised of multiple quantum wells is not easily realizable. Moreover, the Gerard device only operates in a nonequilibrium state established by an optical regeneration source. In other words, the Gerard device must be prompted with pulse light to bring it out of equilibrium and into an unstable state in order to detect infrared light. The sensitivity of the Gerard device depends on the number of optically generated carriers which would decay back to equilibrium via the tunneling dark current or photocurrent induced by the infrared radiation. Thus, the sensitivity of Gerard device suffers as a function of time (i.e., time integrated dark current) and the history of the infrared radiation exposure. In addition, the regeneration requirement of the recombination mechanism of Gerard et al. '992 unduly slows down the response time for emitting visible radiation, thereby decreasing the level of sensitivity of the device.